Apparatus for the preparation of slaked lime solution

ABSTRACT

This invention relates to an apparatus for the continuous preparation of aqueous solution of slaked lime solids with fresh water. The apparatus comprises a main vessel having an inverted truncated cone including a smallest and closed bottom. There is provided a dispenser for supplying material slaked lime solids to said main vessel at or nearly at the top end thereof from above, and a constant headed fresh water reservoir for supplying fresh water to the main vessel at a place selected therein in close proximity to the smallest bottom through a piping. A flow rate adjusting means is arranged in this piping. At least a mechanical or fluid agitator is provided at a close proximity to the vessel bottom. Means for the take-up of the formed product solution in an overflowing manner from the main vessel is provided at the top thereof.

This is a divisional of application Ser. No. 134,051 filed Mar. 26,1980.

BACKGROUND OF THE INVENTION

This invention relates to a method and an apparatus for the continuouspreparation of aqueous solution of slaked lime. Although not limitative,such aqueous solution is broadly used for adjusting pH values in thecity water main lines so as to avoid the formation of iron rust. As analternative use of such aqueous solution, it is used for adding calciumions to artificially regenerated fresh water from sea water.

As the pH-modifier for city water mains, caustic potash has been broadlyused. Recently, however, in place of the potash, slaked lime isincreasingly utilized, because the latter is capable of forming a kindof stable protective layer on the inside wall surfaces of city wateriron pipe lines.

However, there is a grave difficulty in the use of slaked lime which israther difficult to dissolve in water. Inadvantageous local deposits orthe like are liable to form on the inside wall surfaces of the city mainpipe lines. Thus, for using slaked slime for this purpose, a largecapacity of reservation tank is necessary to provide a chance ofsupplying clear aqueous solution of slaked lime.

In last decade, continuously operable apparatus for the production ofaqueous slaked lime solution have been proposed and found theirgradually increased usage.

As a generalized representative apparatus used for the above purpose, itcomprises a tank or vessel, the interior space thereof being, however,separated at a lower level into two by a separating wire net arranged toplace thereon the material slaked lime forming a relatively thick layer.The water for the wanted aqueous slaked lime solution is constantlysupplied preferably at a predetermined constant rate to the place inclose proximity of the tank bottom, while the thus formed saturatedproduct solution is constantly taken out from the preparation tank atthe top thereof.

It has been found that for several initial hours of the lime dissolvingoperation, such plants work substantially satisfactorily. With lapse offurther time, however, the concentration of the product solution willdecrease gradually, on account of such phenomenon that a number ofrelatively large liquid passages are formed through the solids materiallayer and the sizes of these passages will become still larger withfurther continued time of the dissolving operation.

It will thus be seen that the water may pass through these graduallyenlarged liquid passages and without intimate contact with the solidsmaterial.

Another difficulty is caused by clogging of the passage openings of thesolids-supporting wire net as appearing in an extending use of the plantfor at least several months. More specifically, a small amount of sandor the like insoluble particles contained in the material solids willgradually accumulate at the openings of the wire net. On the other hand,the city water may include unavoidably certain amount of iron rust orthe like insoluble solid particles which give rise also to net clogging.Further, a slight amount of free acid(s) may be included in the materialwater which may react with Ca(OH)₂ formed in the solution to CaCO₃,thereby providing liably clogging of the wire net openings.

The invention has its object to provide an efficiently operableapparatus adapted for the continuous formation of normally saturatedliquid solution of slaked lime, without inviting above mentioned severalconventional drawbacks, especially without use of the solids-supportingwire net means, and in a highly stable manner for an extended period oftime.

The invention relates further to a process for the continuouspreparation of the above kind.

SUMMARY OF THE INVENTION

In terms of the apparatus according to this invention, one or more ofmain vessels intended for the desired preparation has its variablesectional area which seen along its central axis. For this purpose, thevessel is so designed and arranged that its main portion including thebottom end has an inverted and truncated hollow cone in which at leastan agitator of either of the mechanical or fluid type is provided at aclose proximity of the closed end of the main vessel. Substantially samelevel or so, at least an outlet opening for supplying fresh water to thevessel is provided so as to its delivery end correctly or at a lowerlevel close to the agitator. The formed product solution is taken outfrom the vessel and at its top end, preferably in an overflowing manner.

As will be clear from the foregoing, the main vessel acting as mainelement of the apparatus according to the invention, the cross-sectionalarea is the smallest at the bottom of the vessel, and it increasesgradually when seen upwards along the central axis of the main vessel.The already or liably deposited solids on the closed smallest bottom endof the vessel may be subjected, in this way, heaviest agitation whichmay be carried into effect, preferably and normally from the beginningof the dissolving operation. However, if the material solids are fine,the agitator may be started only after a certain initial time period,say several hours, counted from the initiation of the dissolvingoperation.

The material fresh water is fed at a place in close proximity of theclosed vessel bottom and preferably at a constant supply rate, whileclear, normally saturated product liquid solution is constantly takenout from the vessel and at the top end thereof.

The agitator may normally started to operate only after certain initialperiod when a decrease in the concentration of product solution willhave been noted.

According to the experiments, the peripheral rotational speed of theagitator, if the latter is of the mechanical one, may normally be lessthan 1 m/sec., most preferably less than 0.5 m/sec. On the other hand,the water velocity supplied at close proximity to the vessel bottom maybe less than 2 mm/sec., most preferably lower than 1 mm/sec, whenmeasured at the top end of the main vessel. In this way, the anexcessively large overall height of the main vessel may be avoidedwithout loss of efficient dissolving performance.

These and further objects, features and advantages of the presentinvention will become more apparent when read the following detaileddescription of the invention to be set forth in connection with theaccompanying drawings, in which

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the arrangement of several components ofthe apparatus according to this invention.

FIGS. 2-4 represent several modifications from the arrangement of thefirst embodiment of the invention shown in FIG. 1.

FIG. 5 is a schematic view of an arrangement in which two main vesselsare used, and indeed, in a seesaw game manner.

FIG. 6 is a schematic diagram showing the variation of concentration ofthe product liquid solution as occurring during an extended time of theoperation, and in a comparative manner with preferred prior arttechnique.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, showing several preferred embodiments ofthe invention, the latter will be described in detail.

In FIG. 1, numeral 1 denotes a main vessel which is so designed andarranged to prepare slaked lime aqueous solution. This vessel 1 isformed with a lower portion 2b which has substantially the shape of aninverted and truncated hollow cone, the inside wall surface thereofserving as a fluidized bed for slaked lime particles, as will be morefully described hereinafter. The upper part 2a of vessel 1 takes theform of a hollow cylinder as shown. An inlet piping 3 opens with itslower end at the top end of the hollow cylinder 2a for introducing thematerial slaked lime. Around the upper end of hollow cylinder 2a, thereis provided an auxiliary overflow-receiving vessel or weir 5, from whichan outlet piping 4 extends for discharging the prepared slaked limesolution accumulated in the auxiliary vessel 5 to a certain reservoir orutilization point of the solution, although not specifically shown.

Numeral 6 represents a water supply piping which opens in the interiorof main vessel 1 and in close proximity to the closed bottom end of thehollow cone portion 2b. The opening end of water supply piping 6 directsdownwards for preventing otherwise possible reverse flow of the preparedslaked lime slurry.

In the main vessel 1, there is provided rotatable agitator vanes 7, theform and arrangement of the latter being most preferably of the type ofa turbine wheel having two or more vanes. These vanes are positionedmost preferably at a slightly higher level than that of the outlet endof water supply piping 6 and at a position as closer possible to theclosed bottom of main vessel 1. The purpose of the turbine-like vanearrangement and the closer as possible positioning of these vanes to thevessel bottom reside in the prevention of any accumulation of solids onthe vessel bottom during the working period and of any disadvantageousinfluence of the agitation to upper part of the liquid phase existing inthe main vessel.

These vanes 7 are attached fixedly at the lower end of a vertically andcentrally arranged drive shaft 100 which extends a small distanceupwards from below through top cover plate 5a of the auxiliary vessel 5and is driven by an electric motor 101 the rotatable part, not shown ofwhich is mounted on top of the shaft 100 and direct-connected therewith,or the motor unit is positioned in the vicinity to the shaft end andconnected therewith by a preferably belt-and-pully transmission system,as a preferred embodiment.

As is only schematically and partially shown at 1a, the upper end ofwall of the hollow cylindrical portion 2a is formed with a plurality ofoverflow-allowable notches, or formed generally in a wavy shape for easyoverflow of the product, or slaked lime solution.

Numeral 13 denotes a slaked lime supply source, only schematically shownin the form of a container bag in FIG. 1, while numeral 14 represents abag filter, these both being connected with each other by a conduit 15which may be at least partially flexible, as is schematicallyillustrated in the same drawing figure. 16 denotes schematically asuction fan which is connected via a suction piping 102 with bag filter14.

The lower outlet end of bag filter 14 is connected with supply piping 3in which a control valve 17 is provided, preferably in the form of arotary valve, as is schematically shown.

Numeral 18 schematically illustrates an air compressor which isconnected via a conduit 103 to bag filter 14 as shown. This compressormay be operated intermittently when dust deposit accumulated on insidesurface of bag filter 14 is desired to be removed.

For the necessary water supply to main vessel 1, a water reservoir 19 isprovided which is so designed and arranged to feed water at apredetermined constant pressure or at a predetermined constant feedrate. A piping 20 coming from city water mainline, not shown, opens withits downstream end in or at the reservoir 19. For maintaining the waterpool in the reservoir 19 at a constant head, the latter is designed asan overflow tank having an overflow piping 21. A water delivery piping22 is connected to the aforementioned water supply piping 6.. A flowcontrol valve 104 and a flow meter 23 are inserted in the deliverypiping 22 as shown.

Substantial part of the inside wall surface of inverted cone-likeportion 2b is provided with a plurality baffle bars or the likeelongated and inwardly projections 24 which serve for accelerating theagitating and upward flow-generating effects of agitator vanes 7 and atthe same time for limiting upwardly moving range of slaked limeparticles within the interior of inverted cone chamber 2b, thus limitingthe upward travel range of the lime particles.

A solids-discharge pipe 25 extends from the main vessel 1 at a lowerlevel close to the agitator 7. This discharge pipe serves for dischargeof insoluble lime particles and contained solid impurities from theinterior space of main vessel 1 preferably continuously and togetherwith a small quantity of liquid prevailing in the vessel 1. For thispurpose, the pipe 25 is provided with an on-off control valve 25a.

In the following, the operation mode of the apparatus according to thisinvention will be more fully described by several numerical examples andby way of reference to FIGS. 1 and 6.

Numerical Example 1

The main vessel 1 shown in FIG. 1 was designed to have an effectivesmallest diameter, as measured at the closed bottom, of 77 mm; and aneffective largest diameter as measured at the open top end, of 146 mm.The main vessel was made of transparent polyvinyl chloride and had itseffective overall height of 2,460 mm.

At first, control valve 104 was opened until water was conveyed fromwater reservoir 19 through pipings 22 and 6 into the interior space ofmain vessel 1 until the latter was perfectly filled with water.

Then, both fans 16 and 18 were operated and control valve 17 was openeduntil 1.43 kg of slaked lime particles (including 50 wt.% of finelydivided slaked lime finer than 63 μ) were sucked off from the containerthrough piping 15 into bag filter. The supplied slaked lime 1.43 kgcorresponds to a material amount for a continuous two-day running of theplant.

For the scheduled continuous operation, the regulator valve 104 was soadjusted that the water velocity measured at the bottom part of mainvessel, having smallest diameter of 77 mm amounted to 0.8 mm/sec. Afterlapse of 24 hours, the thus prepared slaked lime solution wascontinuously taken out from top of the main vessel, while the agitator 7was operated at a peripheral vane speed of 0.6 m/sec. A representativechange in the concentration of the thus obtained Ca(OH)₂ solution isshown by the curve "A" shown in FIG. 6. As will be seen from thischaracteristic curve, the concentration is kept at about 2,000 ppmsubstantially throughout the whole operation.

Numerical Example 2

In this case, the velocity of the supplied water measured nearly at thebottom of main vessel amounted to 0.5-1.0 mm/sec. The rotationalperipheral speed of the agitator vanes amounted to 0.25-1 m/sec. Otheroperational conditions were substantially same as before. Theconcentration took substantially even level of about 2,000 ppmsubstantially throughout the continuous operation.

In this case, the height range as measured at the fluidizing bed forslaked lime particles was 1,600-1,800 mm.

Comparative Experiment 1

As the solution preparing vessel, a hollow cylindrical vessel, made ofpolyvinyl chloride resin, was employed. I.D.: 146 mm; Height: 2,460 mm.

The raw material, slaked lime amounted to 1.3 kg. Same as in theforegoing numerical examples 1 and 2, the agitator was operated, onlyafter 6 hours counted from the initiation of plant operation, and at avane peripheral speed of 0.5 m/sec. The water supply rate nearly at thevessel bottom was set to 0.2 mm/sec, while the product,Ca(OH₂)-solution, was continuously taken out from the vessel top.

The concentration of Ca(OH)₂ was seen to take the variation curve shownat "B" in FIG. 6. As seen, the curve showed a sudden drop upon lapse ofabout 12 hours from the initiation of the plant operation. Thisphenomenon is thus highly disadvantageous for obtaining a substantiallyconstant concentration of the Ca(OH₂)-solution.

Comparative Experiment 2

In this case, a hollow cylindrical vessel, of I.D. 146 mm; and height of2,460 mm, was used and a wire guaze net was fixed to the bottom of thevessel.

As the material, 1.3 kg of slaked lime were used. No agitator was used.

The water was supplied nearly at the vessel bottom as before, and at aflow velocity of 0.2 mm continuously.

The product, Ca(OH)₂ -solution was continuously taken out from vesseltop continuously as before.

The concentration variation in this case is illustrated by the curve "C"shown in FIG. 6. In this case, a substantially linear slow-down wasobserved.

In the modification shown in FIG. 2, the mechanical agitator has beenreplaced by a fluid circulation type agitator. The fluid circulationcircuit is shown at 106, including a circulation pump 8 and a dischargepiping 9. Numerical 107 denotes suction piping.

With this circulating means 106, a part of the liquid contained in themain vessel 1 is sucked at a certain height thereof through suctionpiping 107, after the pump 8 has been operated, and returned again tothe vessel at a level in close proximity of the vessel bottom throughthe delivery piping 9. With this circulation means 106, the liquid andsolid particles are sufficiently mixed. Although the fluidizing bed hasbeen dispensed with, it may be fitted again if necessary. Thesolid-liquid mixture is thus agitated so as to produce a clear Ca(OH)₂-solution. In addition, the water supply piping 6 may be incorporatedwith the circulation circuit with slight modification applied thereto.Although the open end of the delivery piping 9 may be single as shown, amanifold-like multiple inlets can be employed, although not shown.

With the above modified arrangement, enough agitation and liquidturbulent flow can be effectively attained.

Other parts are generally similar with those which have been shown andillustrated in the first embodiment, as being identified with respectivesame reference symbols.

In a further modification shown in FIG. 3, a water ejector 10 has beenemployed for the purpose of liquid-solids agitation. This ejector 10 isinserted in the foregoing water supply 23. An included suction piping 11extends from a certain level of main vessel 1 to the outer portion ofthe ejector. The ejector delivery side is so constructed and arranged inthe similar manner to the water delivery piping 6.

Other main parts are substantially similar with those which have beenembodied in the first embodiment shown in FIG. 1.

As for the operation of the present modification, it can be easilyunderstood from the foregoing description and by consultation withreference numerals shown. If necessary, water ejector system may beprovided in plural for enough agitation and flow-formation in the mainvessel. Fluidizing bed may also be dispensed with. However, if the sizeof main vessel 1 is enough large, it may also be provided, although notspecifically shown and illustrated.

In a further modification from the first embodiment shown in FIG. 1 isillustrated in FIG. 4.

In this case, numeral 12 represents a mechanical agitator of vane typewhich is, however, arranged on a relatively short, horizontallyrotatable drive shaft 100' so as to rotate in a vertical plane. Theshaft 100' carries at its outer end of a driving electric motor 101'which is similar to that shown at 101 in FIG. 1. The vane agitator 12 ispositioned as before, in close proximity to the bottom of the mainvessel 1.

Other parts are similar to those shown in FIG. 1, thus similarrespective symbols are adopted in this embodiment.

The function of the present modified agitator is similar with that shownin FIG. 1. Therefore, no further description in this respect will benecessary.

In each of several embodiments shown in FIGS. 1-4, the slaked limematerial is fed from above to the main vessel 1 preparatorily filledwith fresh water, and the mixture is subjected to strong agitation at aplace in close proximity to the bottom of the main vessel, while freshwater is continuously being fed thereto, again at a place close to thevessel bottom. The operation is carried out until whole of the chargedlime has been substantially completely dissolved, while the product,Ca(OH)₂ -solution is continuously taken out at the vessel top. When theconcentration of the formed solution begins to drop, the operation isprovisionally ceased and an amount of new material of slaked limed ischarged and the operation is restarted.

In the most preferable arrangement is shown in FIG. 5.

In this arrangement, two sets of main vessels 1 and 1' are arranged insuch a manner that these both are operated in a seesaw game mode.

Main vessels 1 and 1' are fitted with water supply pipings 6 and 6',respectively. At a substantially same level therewith, insolublesdischarge pipings 24 and 29' are arranged. At a slightly higher levelthan that of the outlet ends of water supply pipings 6; 6', mechanicalagitators 7 and 7' are provided, respectively.

Numerals 4 and 4' denote product solution discharge pipings throughwhich the formed clear liquid solution can be taken out indirectly fromtop ends of main vessels 1 and 1' through respective overflow-receivingauxiliary vessels 5 and 5', as before. Change-off valves 26 and 26' arearranged in respective product solution discharge pipings 4 and 4', soas to operate the both vessels 1 and 1' in the seesaw game manner.Numerals 3 and 3' represent respective supply inlets to main vessels 1and 1', which are connected through a common feed-rotary valve 17 to thebottom of a common bag filter 14. The lower portion of the latter isconnected by piping to a common container bag 13 which is same in itsfunction as that shown equally with same numeral 13 in FIG. 1.

18 denotes an air compressor as before, while numeral 16 represents asuction fan. The arrangement and function of these units 16 and 18 aresame as before.

Water supply to main vessels 1 and 1' is carried out at a predeterminedspeed or rate and from the common supply water reservoir 19 as before.Numerals 20 and 21 represent similar respective corresponding parts asthose which have been described before.

Although the flow rate-regulator 23 has been shown in somewhat differentrepresentation in FIG. 5 from the foregoing, it is same in substance asbefore.

The water supply pipe line 22 is arranged to be common to the both mainvessels. More specifically, pipe line 22 is connected through change-offvalve 28 to water supply pipings for optionally and alternately selectedwater supply to either main vessel 1 or 1'.

It is now assumed that the first main vessel 1 is under operation. Whenfurther assumed that the change-off valve 26 is so controlled thatproduct solution passes to another water supply piping 6' attributed tosecond main vessel 1', the whole combined plant can be so operated thatproduct solution from the first main vessel will be taken out from tanktop of second main vessel 1'.

In the alternative operation mode, another corresponding control valve26' may be so controlled that product solution from second main vesselmay be continuously taken out from the tank top of first main vessel 1and after passage through respective pipings 4', 27 and 6.

Such series operation may be adopted in a long-run operation and indeedat a later stage thereof. The reason thereof is such that in a long-runoperation of either of main vessels, the product solution may becomelean by virtue of increasingly consumed amount of the charged materialof slaked lime. By adopting such series operation, this possibledisadvantage can be remedied.

Naturally, when either control valve 26 or 26' has been adjusted to itsopposite operating position, the product solution may be discharged assuch in an independent way from another companion main vessel.

The embodiments of the invention in which an exclusive property orprivilege is claimed are as follows:
 1. An apparatus for the preparationof a clear substantially saturated calcium hydroxide solution fromslaked lime and water comprising:a vessel having a lower portion formedby an inverted truncated cone having a closed bottom which defines abottom of the vessel; an upper portion formed by a cylinder having abottom connected to a top of the inverted truncated cone, and having anopen top containing a plurality of periodic indentations around thecircumferential edge; means for removing a substantially clear saturatedcalcium hydroxide solution from the top of the vessel in an overflowingmanner over the circumferential edge; longitudinally extending bafflemeans positioned axially around the circumference of the invertedtruncated cone to cause a slurry in the lower portion to move upwardlyin hindered flow without particle entrainment in the upper portion;inlet means for slaked lime; inlet means for water positioned in closeproximity to the bottom of the vessel, the inlet means directing theflow of water in a downward direction toward the bottom of the vessel;and means for providing agitation positioned near the bottom of thevessel.
 2. An apparatus for the continuous preparation of asubstantially clear and saturated aqueous calcium hydroxide solutionfrom slaked lime particles and fresh water, said apparatus comprising amain vessel having a lower portion formed by an inverted truncated conewith a closed bottom defining a lower zone and an upper portion formedby a cylinder having an open top defining an upper cylindrical zone,dispenser means for supplying slaked lime to the main vessel at ornearly at the top end thereof from above, a fresh water reservoir havinga constant head for supplying fresh water from outside continuously at aconstant feed rate into said vessel through a pipe means in closeproximity to or at above said closed bottom, flow rate adjusting meansarranged in cooperation with the water supply to the main vessel, amechanical or fluid agitator for agitating solids and the fresh water ata same level or slightly higher than said closed bottom, longitudinallyextending baffle means positioned axially around the interior of theinverted truncated cone to cause a slurry in the lower zone to moveupwardly in hindered flow, without particulate entrainment in the upperzone, and means for continuously removing from the top of said vesselsaid substantially clear and saturated aqueous calcium hydroxidesolution by means of overflowing said main vessel.
 3. The apparatus ofclaim 2, wherein the means for removing the hydroxide solution is a weirlocated at the top of the upper portion and the circumferential edge ofthe top of the upper portion has a plurality of notches.
 4. Theapparatus of claim 2, wherein the pipe means for supplying fresh wateris positioned in said vessel to direct the flow of water downwardlytoward the bottom of the vessel.
 5. The apparatus of claim 2, whereinthe flow rate adjusting means for the fresh water is capable ofcontrolling the average ascending water velocity measured at the top endof the vessel so that the water velocity is less than 1 mm/sec.
 6. Theapparatus of claim 2, wherein the agitator is a mechanical agitator. 7.The apparatus of claim 6, wherein the mechanical agitator is a rotaryagitator having a peripheral rotational speed maintained less than 1m/sec.